If I bought a 2506 barrel for a large ring mauser, could it be turned into a small ring 257 roberts? Is there enough shank?

The threads are 0.635" long.
https://www.brownells.com/userdocs/learn/turkishmausers.pdf

Distance from case head to shoulder is 1.948" and 1.727".
The difference is the first part of the set back 1.948 - 1.727 = 0.221"

The shoulder width on the 25-05 is 0.441" and it is 0.429" on the 257R.
The rear of the 25-06 and 257R is 0.470"
The slope of the 25-06 taper is [.47 - .441]/ [1.948 -0.2] = 0.172 inches per inch
To get from a shoulder size of .441 down to .429 it would shrink 0.012"
To shrink 0.012" on a slope of 0.172 inches per inch = .070"
This is the second part of the setback

Adding setbacks 0.221" + 0.070" = 0.291"

Thread percentage remaining = 100% - 0.291"/0.635" x 100% = 54% thread remaining.

A couple months ago I converted a Winchester M70 25-06 barrel to 257 Roberts on an Arisaka. I have plenty of barrel thread left on that project. But....

Some time ago I put Parker hale 300 Win Mag barrels on VZ24s with that much remaining thread.
The two machinists I hunt with [that hate engineers] have sneered at those projects saying they would NEVER do that.
I ask them about force, torque, pressure, steel stress...
I have hunted with them with that rifle, but they are never wrong:
https://en.wikipedia.org/wiki/Selection_bias


Parker Hale Midland rifles barrels from Numrich 10 years ago were $50, used, but bores seemed to be hand lapped... superior to ordinary factory barrels.

I have turned down many large ring Mauser barrels to fit small ring Mauser actions. You need not worry if you know how to run a lathe.
It's easy.

No harder to do than to start with a blank, and faster to finish the job too. Just turn to .980 and thread it normally. Then set the head space by either setting back the shank if you have too much head-space, or deepening the chamber if you have too little. Quite simple.

Turning a LR barrel down to SR isn't hard, but setting it back and rechambering as well, while not hard, makes one wonder why you don't just start from a blank. If you have the ability to recut the threads, and set the barrel back as well as chamber it, then why not just thread and chamber a blank? Seems like there isn't much difference work wise.

^^^^^^^^^^^^^^
Yup.

The only thing you gain is not having to do the contouring. and polishing.

It's pretty much the same job, but less expensive for the lack of the above mentioned time.

And if you start with one of the many "contoured" blanks, it becomes that much easier.

I've seen a lot of botched setback jobs in my day. Most people rely on dimensional drawings and never take into account the actual chamber dimensions. I did rechamber more than a few barrels in my day but always did a chamber cast beforehand to find out exactly what the dimensions were and to see if there were any anomalies present. You'd be surprised to know how many chambers that should "clean up" wouldn't, at least not with what the book says.

Yeah I have seen the same.

My standard method is to cut .030 off the breach end to set everything back that much. Then I either cut the shoulder or the breach face off, to fit the action and bolt perfectly. Now I am assured that the chamber is short in 99% of the cases.

Lastly I install the barrel and then I finish the chamber/headspace with the barrel in the receiver by hand. I set for .002" headspace on a go gauge with slight resistance.

Roberts barrels for SR Mausers are pretty easy to find. Maybe you could trade your barrel for a SR barrel. What is the history of your 25-06 barrel?